Low frequency oscillator employing a pair of u-shaped mechanical vibrators

ABSTRACT

A U-shaped mechanical vibrator having a pair of strip-like vibratory reeds of substantially the same configuration, and a base portion coupling together the pair of vibratory reeds at one end as a unitary structure, the width of each reed being selected greater than the thickness thereof, the vibratory reeds being arranged in a single plane including their surfaces in the widthwise direction in parallel and side-by-side relation, and the pair of vibratory reeds vibrating in anti-phase relation to each other at right angles to the single plane.

United States Patent 1191 Tanaka et a1. 1451 Jan. 9, 1973 [54 LOWFREQUENCY OSCILLATOR 2,942,205 6/1960 McShan ..331/116 M EMPLOYING APAIR OF U-SHAPED 3,269,249 8/1966 Dailey .310/25 x MECHANICAL VIBRATORS3,477,223 11/1969 Tilse etal. ..58/23 TF 3,517,230 6/1970 Lewis etal.... ..84/409 X [75] Inventors: Tetsuro Tanaka, Kyoto; Kiyoshi BmshoTokyo both of Japan Primary Examiner-Roy Lake [73] Assignee: ShlgeruKnkubari, Tokyo, Japan Assistant E iner-Siegfried H. Grimm Filed: Nov. 519-70 Attorney-H111, Sherman, Merom, Gross & S1mpson [21] Appl. No.:87,305 [57] ABSTRACT Related U.S. Application Data A U-shaped mechanicalvibrator having a pair of striplike vibratory reeds of substantially thesame configu- [60] ggg f f it zg i a ggbr gg ig ration, and a baseportion coupling together the pair 1970 3,659,230 of vibratory reeds atone end as a unitary structure, 1 the width of each reed being selectedgreater than the 52 U.S. c1 ..331/37, 58/23 TF, 318/128, thicknessthereof, the vibratory reeds being arranged 331 1 M, 331 15 in a singleplane including their surfaces in the [51] Int. Cl. ..H03b 5/30 wi hwisedirection in parallel and side-by-side rela- [58] Field of Search..331/37, 41, 116 M, 156; tion, and the pair of vibratory reedsvibrating in anti- 310/25; 318/128; 84/409; 58/23 TF; 333/71 phaserelation to each other at right angles to the single plane. [5 6]References Cited 3 Clalms, 26 Drawing Figures UNITED STATES PATENTS3,522,554 8/1970 Tanaka et a1. ..331/1l6 M l "1 /6l3 /ll%1l} /03 1 105,,l28-= l L J 134 #15 1 4 I26 II I32 1 1 1 1 I 1 1 1 1 I g I: 1 H3 115, 17 I I 1 1 l a, I "J l /m I '1 I i i 1 1 1 1' 1 1 1 /20 l 1 1 1 l 1 1 1PATENTED JAN 9 I975 SHEET 03 0F 12 PATENTED JAN 9 973 SHEET UUUF 12 lii.

in Zempemlure PATENIEUJAH 9192s sum 05 0F 12 Iii. 14

PATENTED JAN 9 I975 3,710.2 75

SHEET lEUF 12 LOW FREQUENCY OSCILLATOR EMPLOYING A PAIR OF U-SHAPEDMECHANICAL VIBRATORS REFERENCE TO RELATED APPLICATIONS This applicationis, a division of an application entitled U-Shaped Mechanical Vibrator,Ser. No. 754,416, filed Aug. 21, 1968, and now abandoned in favor of acontinuation application of that same title, Ser. No. 88,507, filed Nov.10, 1970, which issued as U.S. Pat. No. 3,659,230 on Apr. 25, 1972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a vibrator for use in oscillators, mechanical filters or thelike, and more particularly to a vibrator which is small in size, easyto manufacture and suitable for mass production.

2. Description of the Prior Art In the prior art the so-called tuningfork has been proposed as a vibrator but fabrication of such aconventional tuning fork involves an appreciable amount of time and highprecision cannot be expected so that the prior art tuning fork is notsuitable for mass production. Further, miniaturization is verydifficult.

SUMMARY OF THE INVENTION The principal object of this invention residesin the provision of a novel mechanical vibrator which is free from thedrawbacks experienced in the prior art.

The mechanical vibrator of this invention can be produced by punchingprocess or etching of a thin sheet metal, and hence is easy tomanufacture, high in precision and suited for mass production. Further,the present invention allows ease in the production of extremelyminiaturized vibrators.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective viewshowing aconventional tuning fork;

FIG. 2 is a plan view showing one example of a planar tuning fork typemechanical vibrator produced according to this invention;

FIG. 3 is a side view of the vibrator depicted in FIG. 2;

FIG. 4 is a perspective view showing the manner in which the vibrator ofFIG. 2 vibrates;

FIG. 5 is a graph showing loss-frequency characteristics relative to theposition of electromechanical transducer elements;

FIG. 6 is a graph showing loss-frequency characteristics relative to thesize of the electromechanical transducer elements;

FIG. 7 is a connection diagram illustrating one example of an oscillatorcircuit employing the vibrator of this invention;

FIG. 8 is a graph showing its frequency variation rate relative totemperature change;

FIG. 9 is a plan view illustrating, by way of example, one process forthe manufacture of the vibrator of this invention;

FIG. 10 is a plan view showing still another example of the vibrator ofthis invention;

FIG. 1 l is a side view of the vibrator depicted in FIG. 10;

FIG. 12 is a plan view illustrating one example of a planar compoundvibrator consisting of two vibrators of this invention assembled inside-by-side relation;

FIG. 13 is a side view of the planar compound vibrator exemplified inFIG. 12;

FIG. 14 is a connection diagram illustrating one example of the planarcompound vibrator as applied to an oscillator;

FIG. 15 is a graph showing its frequency variation rate relative totemperature change;

FIGS. 16A and 16B are side views respectively illustrating otherexamples of the planar compound vibrator of this invention;

FIG. 17 is a plan view showing one example of a filter employing twoplanar vibrators of this invention;

FIG. 18 is a side view of the filter shown in FIG. 17;

FIG. 19 is a graph showing loss frequency characteristic curves of thefilter of FIG. 17 with the coupling degree of its vibrators being as aparameter;

FIG. 20 is a graphical representation of the relationship of thecoupling degree to frequency deviation;

FIG. 21 is a schematic diagram showing the connections of a signalsource, the filter and output terminals;

FIG. 22 is a graph showing loss-frequency characteristic curves with anexternal resistance being as a parameter;

FIG. 23 is a plan view illustrating another example of the filter ofthis invention;

FIG. 24 is a perspective view illustrating still another example of thefilter of this invention;

FIG. 25 is a plan view illustrating one example of a frequency selectordevice employing a plurality of vibrators of this invention; and

FIG. 26 is a side view of the frequency selector device depicted in FIG.25.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 there is schematicallyillustrated one example of a conventional mechanical vibrator, commonlyreferred to as a tuning fork, which consists of a pair of vibratoryreeds 1a and lb arranged in opposing and predetermined spaced relationand a common base portion 2 interconnecting the reeds. Such a tuningfork is usually produced by machining of a metal block. However, themachining process presents a problem as it requires an appreciableamount of time and introduces a difficulty in obtaining the tuning forkwith high precision and hence the prior art tuning fork is not suitedfor mass production with uniform characteristics. In particular, thisimposes a severe limitation on the construction of small-sized tuningforks.

A detailed description will hereinafter be described in connection witha U-shaped mechanical vibrator of this invention which is free from thedrawbacks encountered in the prior art.

In accordance with this invention the U-shaped mechanical vibratorconsists of a pair of vibratory reeds 3a and 3b formed of resilientmetal sheets in substantially the same configuration and arranged toextend in parallel with each other in the same plane, while being spaceda predetermined distance d and a common plate-like base portion 4 joinedcontiguously to the vibratory reeds'3a and 3b and lying in the sameplane as the reeds, as illustrated in FIGS. 2 and 3. On the vibratoryreeds 3a and 3b there are fixedly mounted in close end portion of thecoupling portion 6' is bent substantially at right angles to be parallelwith the reeds 3a and 3b,.thus providing a mounting portion 7. Themounting portion 7 is secured to a base plate 6 by means of, forexample, a screw 8 by which the U-shaped vibrator is mounted on the baseplate 6. The U-shaped vibrator may be formed, for example, by punching athin-metal sheet to have the vibratory reeds 3a and 3b, the base portion4, the coupling portion 6 and the mounting portion 7 as a unitarystructure. The vibrator may be made of a resilient material having smallcoefficient of thermal expansion such, for example, as ELINVAR. Withsuch a vibrator (hereinafter referred to as a planar tuning fork) it hasbeen ascertained that the vibratory reeds 3a and 3b vibrate opposite inphase in a direction at right angles to their surfaces (in this case thewidth W, of the vibratory 'reeds 3a and 3b is selected well greater thanthe thickness t thereof). Namely, the vibratory reeds 3a and 3b vibratein such a mode that they first swing away from their common referenceplane in opposite directions and then back to the reference plane, asdepicted in FIG. 4, in which manner the vibratory reeds 3a and 3brepeatedly vibrate. r

The resonance frequency f of the planar tuning fork is given by thefollowing equation:

f=(1/21r)=(a,,2/F) (z/V 12) /(E/p) (1) where am is a coefficientvaryingwith vibrating conditions of the tuning fork and is 1.8751 in the caseof basic vibration, namely m being equal to l, l is the length of thevibratory reeds 3a and 3b in their longitudinal direction (refer to FIG.2), t their thickness (refer to'FlG. 3) p the density of their materialand E its Youngs modulus.

For example, in the case where lis 18.5 mm., t is 0.5 mm., the width W,of the vibratory reeds 3a and 3b is 1.8 mm. and the distance d, betweenthe vibratory reeds is 1 mm., the resonance frequency f of the tuningfork is 1,015 c/s from the above equation 1.

Now, a discussion will be made in connection with the influence exertedon the vibration of the vibratory reeds 3a and 3b by the position of thepiezoelectric elements 5a and 5b. With the distance d, from thepiezoelectric elements 5a and 5b to a demarcation line 9 between thevibratory reeds 3a and 3b and the base portion 4 being 1 mm., and 1 mm.,the elements a and 5b each being formed of PZT to have a length L of 3.0mm., a width W of 1.7 mm. ancla thickness of 0.25 mm., the insertionloss'vs. frequency characteristics are respectively given as indicatedby curves 10, 11 and 12 in FIG. 5 in which the ordinate represents lossin dB and the abscissa frequency in c/s and d, is employed as aparameter. As will be apparent from the graph, when d, 1 mm. theresonance frequency f is 1,015.5 c/s nearly equal to the aforementionedcalculated value and when d, 0 and 1 mm. the resonance frequencies arerespectively as high as 1,041.5 c/s and 1,044.5 c/s.

This is considered to result from the fact that the piezoelectricelements 5a and 5b act as stiffness on the vibratory reeds 3a and'3b,not as mass. The Qs of the planar tuning fork in the above three casesare 1,128 in the case of d, being 1 mm., 1,160 in the case of d, being 0and 1,158 in the case of d, being -1 mm., and the Q in the first case isa little lower than the others. This implies that the Q lowers as thepiezoelectric elements approach the side of the vibratory reeds 3a and3b of large amplitude, that is, their free ends. Consequently, the in-'sertion loss is most minimized with d, being 1 mm. The ratio of anoutput voltage to an input voltage, that is, the coupling factor is alittle greater when d, 1 mm. The foregoing numerical values are given inthe following table 1.

TABLE 1 Position Resonance Q Insertion Coupling (1, mm frequency (c/s)loss (dB) factor A, "1015.5 1128 0.7 0.029 B 1041.5 1160 2.0 0.027 C-l1044.0 1158 2.0 0.027

FIG. 6 is a graph showing the influence exerted upon the insertion lossby changing the size of the piezoelectric elements 5a and 5b, in whichthe ordinate represents the loss in dB and the abscissa frequency inc/s. In the figure the curve 13 indicates a case of the piezoelectricelements each having a length of 3 mm. and a width of 1.7 mm. and thecurve 14 a case of the elements having a size of a length of 1.3 mm. anda width of 1.2 mm. It appears from the graph that the resonancefrequencies are approximately equal to 1,015.5 c/s and Qs are alsosubstantially equal to 1,128 but that the insertion losses are 0.7 dBand 3 dB. Further, the coupling factors are 0.027 and 0.025, namely thesmaller piezoelectric elements slightly lower the coupling factor.

It is preferred that the distance d between the vibratory reeds 3a and3b be smaller than the width W of each reed, for example, less thanone-half thereof. This is because of the fact that with the distance dbeing greater than the width W of the reeds 3a and 3b, torsion or twistis yielded in the base portion 4, that is, the coupling portion of thereeds which is likely to cause a variation in the resonance frequency.In addition, the width W, of the vibratory reeds is selected smallerthan the length l of the base portion 4 in the lengthwise direction ofthe vibratory reeds and the length I; is selected, for example, morethan 1.5 times as great as the width W The reason is that the length I,smaller than the width W, causes an increase in vibration rendered tothe mounting portion 7 through the coupling portion 6 from the baseportion 4. Where the piezoelectric elements 5a and 5b are deposited onthe same side of the vibratory reeds 3a and 3b, an input signal fed toeither one of the piezoelectric elements and an output signal obtainedfrom the other piezoelectric element are opposite in phase.Consequently, an output signal in phase with the input signal may beobtained by the use of piezoelectric elements of opposite polarities orby depositing the piezoelectric elements on different surfaces of thevibratory reeds. I FIG. 7 illustrates one example of a self-excitedoscillator employing the planar tuning fork of this invention describedabove, which may be provided by the same connections as a self-excitedoscillator using a conventional tuning fork except the use of the planartuning fork of the present invention, as depicted in the figure. Namely,an amplifier 21 consisting of transistors 19 and 20 of cascadeconnection is provided and connections are made such that one portion ofthe output from the output terminal of the amplifier 21 is applied tothe piezoelectric element, for example 5a of the vibratory reed 3a ofthe planar tuning fork and an output obtained by the other piezoelectricelement 5b of the vibratory reed 3b is fed to the input side of theamplifier 21, that is, to the base of the transistor 19, thus providingan oscillator having an oscillation frequency determined by theresonance frequency of the planar tuning fork. In FIG. 8 there isdepicted the frequency variation rate Af/f relative to temperaturechange (C) in the above case, the ordinate representing the frequencyvariation and the abscissa temperature. This graph" was obtained in thecase where only the tuning fork was subjected to temperature change.From the graph it appears that the frequency variation is 1 X 10 deg. ina range of about 19C to 59C, which indicates an excellentcharacteristic.

Although the foregoing has stated that the planar tuning fork of thisinvention may be produced by press work (punching process), it may alsobe made of one base plate by means of chemical etching techniques. Thatis, grooves 16 are formed in a thin sheet of ELINVAR by chemical etchingin a manner to leave the vibratory reeds 3a and 3b, the base portion 4and the coupling portion 6', as shown in FIG. 9. In this case, it ispreferred to form a groove 17 by chemical etching at a distance from thecoupling portion 6' on the opposite side from the vibratory reeds. Withphotoetching techniques used in the manufacture of semiconductordevices, smalIed-sized tuning forks can be produced with high precision,and the planar tuning fork thus produced is suitable for use with, forexample, semiconductor integrated circuit devices.

In the foregoing example the piezoelectric elements are used aselectromechanical transducer elements but may be substituted withelectromagnetic transducer elements. Namely, instead of mounting thepiezoelectric elements 5a and 5b on the planar tuning fork,electromagnetic units 18a and 18b each consisting of a core and a coilwound thereon are disposed opposite the vibratory reeds 3a and 3b in thevicinity of the free ends where their amplitude of vibration is great.With an exciting signal being fed to, for example, the electromagneticunit 18a, the vibratory reed 3a is driven to vibrate and the vibratoryreed 3b is thereby driven through the base portion 4, by which anelectric signal of the same frequency as the signal fed to the unit 18ais obtained from the electromagnetic unit 18b.

As has been described in the foregoing, this invention enables massproduction of planar tuning forks of high precision and uniformcharacteristics without requiring such a troublesome machining of ametal block.

In FIG. 12 there is illustrated another example of this invention inwhich a plurality of, for example, two planar tuning forks such asdepicted in FIG. 2 are jointed together at their base portions withtheir surfaces being flush with each other. A detaileddescription willhereinbelow be given of this example. Reference numerals 101 and 102indicate two planar tuning forks, which are assembled together in thefollowing manner. That is, vibratory reeds 103a, l03b and 103a,, 103b,are disposed substantially in parallel at moderate intervals with theirplanar surfaces being substantially flush with one another, and couplingportions 106' and 106, extending from the base portions 104 and 104, onthe side remote from the vibratory reeds in parallel relation theretoare secured to a common support 113, thereby providing an assembly ofthe tuning forks. In this case the base portions 104 and 104, of theplanar tuning forks 101 and 102 are jointed together through a jointportion 109. Reference numerals 105a, 105b, 105a and l05b designatepiezoelectric elements fixed mounted on the vibratory reeds of theplanar tuning forks, which are identical with those exemplifiedin FIG.2. The planar tuning forks thus assembled together are mounted on a baseplate 106 by clamping their common support 113 onto the base plate 106by means of a screw 116 inserted through a hole 115 bored in the support113. It is a matter of course that such a compound tuning forkconsisting of the two planar tuning forks 101 and 102 depicted in FIG.12 may be produced from a sheet metal by means of punching or etching inthe same manner as that described above with FIG. 2.

The resonance frequency of each tuning fork is determined by theequation 1 as previously described but the frequencies of the two planartuning forks are rendered different from each other by selecting thelength of the vibratory reeds of either one of the tuning forks to besmaller or greater than that of the reeds of the other. In FIG. 12 thevibratory reeds 103a, and 103b, of the tuning fork 102 are shorter thanthose 103a and l03b of the other tuning fork 101. It is preferred thatthe vibratory reeds of the same tuning forks, that is, 103a and 1031; or103a, and 103b are identical in shape with each other.

Assuming that the widths of the vibratory reeds of the planar tuningforks 101 and 102 are taken as a, and a the distance between thevibratory reeds 103a and 103k is b,, the distance between the reeds 103aand 103b, is b and the lengths of the reeds 103a, 1013b and 103a,, 103b,are I, and I respectively, a 11,, a, and b are selected such that a b,and a b This is because of the fact that b, or b exceeding a or aproduce torsion in the base portion 104 or 104' to cause a change in theresonance frequency. Further, if the lengths of the base portions 104and 104 in the lengthwise direction of the vibratory reeds are taken asc, and c (c 0 in the figure), c and c are selected greater than thewidths a and a of the vibratory reeds. With being smaller than a, or a,,vibration of the base portion 104 or 104, in the direction of thecoupling portions 106 and 106 increases, which is undesirable as setforth above. In order to provide the coupling portion 109, a slit 116'is formed in the jointed portion of the base portions 104 and 104, onthe side of the vibratory reeds, in which case the depth W of the slitis selected great enough to permit the tuning forks 101 and 102 tofunction independently from each other.

For instance, a typical size is such that a, a, 1.8 mm., b, b, 1.0 mm.,W 1.6 mm., the thickness t of the vibratory reeds 103a, 103b and 103a,,103b; 0.5 mm. and the distance d between the adjacent vibratory reeds103b and 103a, 1.0 mm. In such a case, the

resonance frequencies f; and f, of the planar tuning forks 101 and 102are 1,377 c/s and 1,299 c/s respectively. To electrically drive thetuning forks 101 and 102 to-obtain electric signals therefrom,piezoelectric elements 105a,-105b and 1050,, 105b, of, for example, PZTmay be deposited by an adhesive binder on the vibratory reeds 103a, 103band 103a, and 103b, on the side of the base portions 104 and 104 g Inthe example shown in FIG. 12 the planar tuning forks 101 and 102 areformed as a unitary structure but each of them performs the function ofsubstantially an independent tuning fork. Consequently, it is possiblethat separate oscillators having the tuning forks as the referencefrequency sources are provided and adapted to obtain an output of afrequency corresponding to a difference in their oscillation outputs, sothat an oscillation output of low frequency can be obtained withrelatively small tuning forks. FIG. 14 is a connection diagramillustrating one example of such construction. In the figure referencenumeral 124 indicates an amplifier consisting of transistors 119a and119b, the transistor 119a having its collector connected to a powersource 120 and its emitter grounded through a resistor 121 and connectedto the base of the transistor 119b and the transistor 11% having itscollector connected to the power source 120 through a resistor 122 andits emitter grounded through a resistor 123. Reference numeral 125designates an oscillator circuit having incorporated therein theamplifier 124. Namely, the input end of the amplifier 124, that is, thebase of the transistor 119a is connected to, for example, thepiezoelectric element 105b of the vibratory reed 103b of the planartuning fork 101, and the output side of the amplifier, that is, thecollector on the transistor 1 19b is connected to the piezoelectricelement 105a of the vibratory reed 103a, by which the vibratory reed103a is driven to drive the vibratory reed 1013b and the transistor 11%is driven by an output of the piezoelectric element 105b of thevibratory reed 103b to permit oscillation of the oscillator circuit 125at the resonance frequency of the planar tuning fork 101. In a similarmanner an amplifier 130 is constituted with transistors 126a and 126b,and the input side of the amplifier 130, that is, the base of thetransistor 126a is connected to the piezoelectric element 105b, of theother tuning fork 102 and the output side of the amplifier, that is, thecollector of the transistor 126b is connected to the piezoelectricelement-105a,, thus providing an oscillator circuit 126 oscillatingatthe resonance frequency of the planar tuning fork 108. Further, theseoscillator circuits 125 and 126 are interconnected and their outputs arefed to a frequency converter. In the figure the oscillators 125 and 126are coupled together by the mechanical coupling of the planar tuningforks 101 and 102 through the coupling portion 109, under whichconditions when the oscillator, for example, 126 is main, theoscillation frequency f, of the oscillator 126 isamplitude-modulated atthe oscillation frequency f,

of the oscillator 125. Accordingly, the output of the oscillator 126,that is, the collector output of the transistor 126b is fed through alow-pass filter 131 to an amplifier 133 consisting of a transistor 132of emittergrounded connection, from an output terminal 134 of whichamplifier can be obtained a signal f, -f, =f corresponding to adifference in the oscillation frequencies of the oscillators and 126. Itis also possible in this case that the respective outputs of theoscillators 125 and 126, that is, the output of the transistor 119b andthe collector output of the transistor 126b are separately applied tothe common frequency converter circuit to obtain a beat frequencytherebetween. In order to facilitate coupling of the planar tuning forksthrough the coupling portion 109 for obtaining an amplitudemodulatedsignal, it is preferred that in the planar tuning fork 102 of the mainoscillator the outer vibratory reed 103b, (on the opposite side from theplanar tuning fork 101) is a drive side and the inner reed 103a, apickup side and that in the other planar tuning fork 101 the innervibratory reed l03b is a drive side and the outer reed 103a a pickupside. The planar tuning forks 101 and 102 for producing the referencefrequencies of the oscillators 125 and 126 for obtaining a beat signalare produced as a unitary structure by'punching of a resilient sheet ofmetal, forexample, ELINVAR. Consequently, the frequency vs. temperaturecharacteristics of the two oscillators are substantially the same andfurther since a difference frequency is obtained, a

beat frequency output remarkedly stable in tempera-- ture can beproduced. In FIG. 15 there is illustrated frequency change ratio Af/frelative to temperature change in the case where only the planar tuningforks 101 and 102 are subjected to temperature change. It appears fromthe graph that the temperature coefficient is substantially zero in atemperature range of 19C to 65C. In the illustrated example f, 1,377c/s, f 1,299 c/s and f 78 c/s at a temperature of 20C. Since a signalcorresponding to the difference in the oscillation frequency between thetwo oscillators is taken out as described above, even if the planartuning forks 101 and 102 are miniaturized, a low-frequency signal can beobtained. In the prior art, a tuning fork oscillating, for instance, at78 c/s is bulky and is difficult to drive. In the example depicted inFIG. 14 the piezoelectric elements are employed as electric transducerelements but they may be replaced with, for instance, electromagnetictransducer units or electrostatic transducer units, as will hereinbelowbe described with FIG. 16. That is, as depicted in FIG. 16A anelectromagnetic unit consisting of a magnetic member and a coil woundthereon is disposed opposite the vibratory reed of the planar tuningfork, or fixed electrode 136 is placed as the electrostatic transducerunit in opposed relation to the vibratory reed, as

illustrated in FIG. 16B, in which case a high DC voltage source 137 isapplied between the electrode 136 and the vibratory reed while at thesame time applying or taking out an AC signal. It will be apparent thatall or tuning forks, in which case a difference between the vibrationfrequencies of two tuning forks is first obtained and then a differencebetween the resulting difference frequency and the vibration frequencyof the remaining tuning fork is obtained. Further, it is easy to producean oscillator having oscillation frequencies corresponding to thedifferences in the vibration frequencies of more than four tuning forks.

In FIGS. 17 and 18 planar tuning forks of this invention such asdepicted in FIG. 2 are mechanically coupled together in the same manneras in the example shown in FIGS. 12 and 13. The similar parts to thosein FIGS. 12 and 13 are identified by the similar reference numerals andno detailed description will be repeated for the sake of brevity. Inthis case the vibratory reeds 1030, 103b, 103a and 103b, of the planartuning forks 101 and 102 are substantially equal in length I to oneanother. As driving and detecting elements of a filter, piezoelectricelements of, for example, PZT are used but in the present example thepiezoelectric elements are mounted on the two outer vibratory reeds 103aand 103b, of the tuning forks 101 and 102 in proximity to the baseportion 104 and 104 thereof, as indicated by 105a and 105b,.

The resonance frequency f of these planar tuning forks 101 and 102 isgiven by the aforementioned equation 1;

For example, where L, 1.85 cm. and t= 0.05 cm., the resonance frequencyf is 10,150 c/s.

Applying an electrical signal to the piezoelectric element 105a of theplanar tuning fork 101, the vibratory reed 103a is thereby vibrated,which leads to driving of the vibratory reed 103b in anti-phase relationto the reed 1030, thus rendering the planar tuning fork in its drivencondition. This applies vibration to the planar tuning fork 102 throughthe coupling portion 109 to cause its vibratory reeds 103a, and 1031b tobe driven at the same time, with the result that an electrical signal istaken out from the piezoelectric element 105b, of the vibratory reed103b, of theplanar tuning fork 102. In such a case, the planar tuningforks 101 and 102 are caused to vibrate only by a signal having aparticular frequency equal to their resonance frequency, and they hardlyvibrate at other frequencies. Therefore, a filter having a pass bandcorresponding to. the oscillation frequency of the planar tuning forkscan be provided.

By the way, if the width of the vibratory reeds of each of the planartuning forks 101 and 102 is taken as a and the distance between adjacentvibratory reeds is b, b is selected to be about one half of a. When b isgreater than a pseudo-vibration is caused in the base portions 104 and104 to shift the resonance frequency. In addition, the length c of thebase portion 104 or 104, in the lengthwise direction of the vibratoryreeds is selected greater than the width a of the vibratory reeds, forexample, about 1.5 times as great as the width 0. This is because of thefact that when is smaller than a vibration is much transmitted to thesupport 113 through the coupling portions 106' and 106,. It is preferredto locate the piezoelectric elements 105a and 105b, a little further tothe free end of the vibratory reeds than the demarcation between thereeds and the base portions 104 and 104,. In this case theresonancefrequency of the planar tuning forks becomes approximatelyequal to the aforementioned equation 1. There is a tendency thatshifting of the piezoelectric elements toward the free ends of thevibratory reeds causes their resonance frequencies to deviate higher.Now, the coupling portion 109 will be discussed. If the distance fromthe coupling portion 109 to the demarcation between the vibratory reedsl03b and 103a, and the base portions 104 and 104, is taken as W, a losscharacteristic curve with W varying as a parameter is as shown in FIG.19, in which the ordinate represents loss in dB and the abscissafrequency in c/s. That is, curves 12, 13 and 14 respectively indicatethe cases of W being 1.50 mm., 1.56 mm. and 1.62 mm. From the graph itappears that a decrease in W tightens the coupling of the two planartuning forks 101 and 102 and causes the characteristic curve to bedouble-humped and widens its pass band, for example, up to about 8 c/sin the graph. In the case of the curve 13 the pass band is approximately5 c/s. With the lowering of the coupling of the planar tuning forks,that is, with an increase in W, the characteristic curve becomes to besubstantially single-humped and its pass band width becomes 2.5 c/s. Thedepths of the troughs of the curves 12, 13 and 14 are respectively 8 dB,5 dB and 1 dB. It will be understood from this that the band width canbe widened by decreasing W to increase the coupling of the planar tuningforks 101 and 102 and that the band width can be narrowed by increasingW to decrease the coupling. In the above example the values of a, b, ..tare those previously mentioned and the piezoelectric elements of PZT areemployed and are of a size of 3 X 2 mm Further, it will be seen that therelation between the frequency difference of the peaks of thecharacteristic curves and W is such that an increase in W causes alinear decrease in the frequency difference as shown in FIG. 20, theabscissa representing W in mm. and the ordinate the frequency differencein c/s.

The filter characteristic of the filter such as depicted in FIGS. 17 and18 can be improved by input and output resistances. That is, as shown inFIG. 21, a signal is applied to the filter 215 of this invention from asignal source through a resistor 217, namely the signal is fed to thepiezoelectric element 105a of the filter 215, and a resistor 218 isconnected between the output side or the piezoelectric element 105b, andground, and output terminals 219 are led out from the both ends of theresistor 218. This provides a maximum output when the resistance value Rof the resistor 218 on the output side is (1/21rfc c, being thecapacitive component of the piezoelectric element PZT. Where theresonance frequency is 1,000 c/s and the capacitive component 0 of PZTis 2,000 PF, R l megohm is a maximum output. In FIG. 22 there isillustrated a graph showing loss frequency characteristic curvesobtained with the resistance values of the resistors 217 and 218 beingaltered, the ordinate representing loss in dB and the abscissa frequencyin c/s. The curve 20 in the graph indicates the case where theresistance value R, of the resistor 217 is zero and R is infinite, inwhich case the depth of the trough between peaks of the curve 20 is 6.5dB and the insertion loss at the peaks is approximately zero. When R, 0and R 1 M9, the insertion loss at the peaks is about 9 dB but thecharacteristic curve becomes nearly single-humped and the depth of thetrough is 3 dB. Further, when R, is 1 M0, the insertion loss is the sameas in the above but the characteristic curve becomes furthersingle-humped and the depth of the trough is about 1 dB. The band widthdoes not vary with the resistance value R but lowers from 5 cls to 4.5cls when R, is altered from to l MQ.

As has been described above, this invention permits of fabrication ofthe tuning fork filter by means of punching a sheet metal and allowsease in mass production of miniature and highly precise filters. Theband width can be adjusted by controlling the coupling degree of thecoupling portion 109 of the two planar tuning forks 101 and 102.

The filter described above exhibits excellent temperature characteristicsuch that the frequency change ratio relative to a temperature change is-l X '10 or so. In addition, the cut-off characteristic of the filter isalso excellent, as will be seen from the aforementioned losscharacteristic.

The cut-off characteristic can be enhanced by further connecting theplanar tuning forks in side-byside relation, as exemplified in FIG. 23.In the figure a planar tuning fork 103 identical with the forks 101 and102 is interposed therebetween, and these tuning forks may be producedby punching of a sheet metal. In this case all the planar tuning forksare coupled integral with the support 113 through a coupling portion106, 1 extending from a base portion 104, of the intermediate structureof the planar tuning forks. With the three planar tuning forks 101, 102and 103 being coupled together in side-by-side relation, a filter ofexcellent selectivity-can be obtained. It is possible to connect moreplanar tuning forks in side-by-side relation.

In the above example, the planar tuning forks are arranged with theirvibratory reeds lying substantially in one plane but such arrangement isnot always necessary. It is sufficient only to dispose a pair ofvibratory reeds of at least one planar tuning fork in one plane. Forexample, it is possible that a plurality of planar tuning forks areassembled together by suitable coupling member in a manner to arrangetheir pairs of vibratory reeds one over another in spaced relation, asexemplified in FIG. 24. In the figure, four planar tuning forks 301,302, 303 and 304 each having their two vibratory reeds in one plane areemployed and these planar tuning forks are assembled together to arrangetheir pairs of vibratory reeds one over another in opposed andmoderately spaced relation, and base portions 304, 304 304, and 304 ofthe planar tuning forks 301, 302, 303 and 304 are extended in adirection remote from the vibratory reeds. The extended base portionsare respectively put between block members 30b,-30b,, 30b, and 30b, andbonded together, and coupling members 309 309, and 309, are eachinterposed between adjacent planar tuning forks at a place on the baseportions 304, 304 304, and 304 or on the vibratory reeds close to theportion, thus interconnecting the planar tuning forks. In this case itis preferred that the planar tuning forks are each produced by punchingof a sheet metal and assembled together so as to ensure uniformity oftheir temperature characteristic. Also in this case piezoelectricelements are mounted on the vibratory reeds of the uppermost andlowermost tuning forks.

In the examples above described with FIGS. 17 23 and 24 the filtersemploy the plate-like planar tuning forks and hence they can beminiaturized in construction. Further, the planar tuning forks of highprecision and uniform characteristics can be mass produced by punchingprocess to ensure the fabrication of excellent filters. The use of thephotoetching techniques employed in the manufacture of semiconductorsallows ease in the production of small-sized planar tuning forks,'whichleads to further miniaturization of the filters.

In the above examples the piezoelectric elements are used as driving anddetecting elements of the filters but they may be replaced withelectromagnetic or electrostatic elements. It is of course possible toemploy these three electromechanical transducer elements in combination.

FIGS. 25 and 26 illustrates a frequency selector unit which isapplicable to an alarm device, a frequency analyzer or the like and inwhich a plurality of planar tuning forks of this invention are employedand signals are selectively picked up according to their frequenciesfrom many input signals of different frequencies.

In the figure reference numerals 401, 402, 403, 40n respectivelydesignate planar tuning forks such as exemplified in FIG. 2. The tuningforks 401, 402, ..40n are respectively provided with a pair of vibratoryreeds 401a and 401b, 402a and 402b, 40m and 40nb, in exactly the samemanner as in the foregoing examples. All the planar tuning forks 401,402, ..40n are assembled together as a unitary structure at their baseportions 404,, 404 404,, through coupling portions 409,, 409 ..409,, insuch a manner that their vibratory reeds 401a, 401b, ..40na, 40nb maylie in the same plane at certain intervals. Free ends of couplingportions 406,, 406 ..406,,' extending from the central portions of thebase portions of the tuning forks in a direction opposite to thevibratory reeds are bent substantially at right angles and are securedto a support 413 parallel to a base plate 406 which support 413 extendssubstantially in parallel with the vibratory reeds. Further,electromechanical transducer elements such, for example, aspiezoelectric elements 4015a and 4015b, 4025a and 4025b, ..40n5a and40n5b are deposited, by means of an adhesive binder, on the vibratoryreeds 401a and 401b, 402a and 402b, .....40na and 40nb of the planartuning forks 401, 402, ..40n in proximity to the base portions 404,,404,, ..404n thereof. The planar tuning forks 401, 402, 403, ..40n aredifferent in length of the vibratory reeds so as to obtain differentresonance frequencies. The resonance frequencies f of the planar tuningforks are given by the aforementioned equation 1 as in the foregoingexamples:

In FIG. 25 the planar tuning forks 401, 402, 403, .....40n are adaptedsuch that their resonance frequencies f f f ..f,,, gradually lower.Namely, the lengths of the vibratory reeds of the planar tuning forksare rendered sequentially greater. In-each tuning fork the width a ofits vibratory reeds is selected greater than the space b between thereeds, for example, ((1 12) z b With I); being greater than a; torsionalvibration is yielded in the base portion to change the resonancefrequency f and hence a stable resonance frequency cannot be obtained.Further, the length 0,, of the base portion in the lengthwise directionof the vibratory reeds is selected greater than the width a for example,l.5a z When c,, is smaller than a vibration of the vibratory reeds ismuch transmitted from the coupling portion to the support to cause loss.As in the foregoing examples, the vibratory reeds of each planar tuningfork vibrate in anti-phase relation to each other at right angles totheir plane. That is, they vibrate in such a manner that they first goaway from their'plane in opposite directions and then back to the plane.

In FIG. 25 the coupling portions 409,, 409 .....409,, are formedcontiguous to adjacent ones at the ends of the base portions of theplanar tuning forks remote from the vibratory reeds but coupling membersmay be formed contiguous directly to the vibratory reeds in proximity tothe base portions thereof. These planar tuning forks, their couplingportions and the support may be produced by punching process of a sheetof a resilient alloyed metal such as ELINVAR. That is, they are producedsuccessively from one material without interruption. Where thepiezoelectric elements of PZT are employed as the electromechanicaltransducer elements, it is preferred to locate them on the vibratoryreeds of the planar tuning forks in close but definitely spaced relationto the base portions so as to obtain resonance frequencies nearly equalto calculated values. Further, it is also preferred to form the couplingportions 409,, 409 409 ..409,, as small as possible to ensure theelimination of interference between adjacent planar tuning forks. Thesupport 413 for mounting the planar tuning forks on the same base plate406 may be formed in common to all the tuning forks but may be providedseparately for each of them.

The electromechanical transducer elements 4015a, 4025a, 4035a, ..40n5a,each being one of the elements in pairs, are connected to a commonsignal source 411. Namely, signals are impressed from the common signalsource 41 1 to the planar tuning forks in parallel relation. While,signals are separately led out from the other electromechanicaltransducer elements 4015b, 4025b, 4035b, ..40n5b. In the figuretransistors 412a, 412b, 412e, ..412n are provided each for each of theplanar tuning forks 401, 402, ..40n and are connected, for example, inam emitter grounded manner, the collectors of the transistors beingrespectively connected to oneend of a power source E through resistors4130, 413b, 413c ..4l3n and the bases being respectively connected tothe transducer elements 4015b, 4025b, ..4025n of the output side of theplanar tuning forks. In addition,

output terminals 4140, 414b, 4140, ..414n are connected to thecollectors of the transistors.

With such an arrangement as shown in FIG. 25, upon application of asignal having a frequency f from the signal source 41 1 to the selector,only the planar tuning fork 401 vibrates to thereby feed a signal to thetransistor 412a, providing a signal of the frequencyf at the outputterminal 414a. In a similar manner, only the planar tuning fork'having aresonance frequency equal to the frequency of a signal fed from thesignal source vibrates to provide a signal output at the terminalcorresponding to the vibrated planar tuning fork, by which the frequencyof the signal from the signal source 411 can be known. Even if aplurality of signals of different frequencies are simultaneously appliedfrom the signal source 411 to the selector, only the planar tuning forkscorresponding to the particular frequencies vibrate to produce signalsat the corresponding output terminals. Thus, the selector unit of thisexample enables detection of the frequencies of the signals from thesignal source 411 at the output terminals. Consequently, the unit can beused as a frequency analyzer unit, an alarm device or the like. In thecase of the alarm device, a plurality of alarm signal sources placed atdifferent locations are adapted to raise alarm signals of differentfrequencies, by which detection of an output produced at any of theoutput terminals 414a, 414b, 4140, ..414n enables location of the sourceof the alarm signal being raised.

As described above, the selector unit employing the planar tuning forksof this invention enables discrimination of signal frequencies and inthis case the planar tuning forks 401, 402, 403, ..40n are formed as aunitary structure with their vibratory reeds lying in substantially thesame plane, so that the entire structure can be produced by punchingprocess of one sheet metal to facilitate the fabrication. Further, theseplanar tuning forks are of substantially the same temperaturecharacteristic and hence a selector unit stable in temperature can beobtained. Even if the temperature characteristic is a little altered,the alterations of the planar tuning forks are rendered uniformly in thesame direction and hence can easily be compensated for. If the punchingtemplate of high precision is made, highly precise selector unit can bemass produced.

In the foregoing the piezoelectric elements are used as theelectromechanical transducer elements but they may be substituted withelectromagnetic or electrostatic transducer elements. In addition, ahigh precise selector unit can be produced as described above, so thatif input signal frequencies are extremely close to one another, theinsertion loss can be held low because of sharp resonance frequencycharacteristic of the planar tuning forks. For example, when a, 1.8 mm.,b 1.0 mm., c 1.35 mm., d,,= 18.5 mm. and t 0.5 mm., the resonancefrequency is 1,015.0 c/s and Q is more than 1,100 and insertion loss canbe held lower than 0 to several dB.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts of thisinvention.

We claim as our invention:

l. A low frequency oscillator comprising two U- shaped mechanicalvibrators, each including a pair of strip-like vibratory reeds ofsubstantially the same configuration and a base portion couplingtogether the pair of vibratory reeds at one end as a unitary structure,the width of each of the vibratory reeds being selected fully greaterthan the thickness thereof, the pair of vibratory reeds being arrangedin spaced and parallel relation to each other with their surfaces lyingin substantially the same plane and vibrating in anti-phase relation toeach other at right angles to the plane which includes their surfaces,the length of the vibratory reeds of one of said

1. A low frequency oscillator comprising two U-shaped mechanicalvibrators, each including a pair of stRip-like vibratory reeds ofsubstantially the same configuration and a base portion couplingtogether the pair of vibratory reeds at one end as a unitary structure,the width of each of the vibratory reeds being selected fully greaterthan the thickness thereof, the pair of vibratory reeds being arrangedin spaced and parallel relation to each other with their surfaces lyingin substantially the same plane and vibrating in anti-phase relation toeach other at right angles to the plane which includes their surfaces,the length of the vibratory reeds of one of said U-shaped mechanicalvibrators being selected shorter than that of the other vibrator so thatsaid vibrators vibrate at the respective frequencies f2 and f1, a firstelectromechanical transducer element fixedly mounted on one of thevibratory reeds of said one U-shaped mechanical vibrator and a secondelectromechanical transducer element fixedly mounted on the othervibratory reed of said one U-shaped mechanical vibrator, a firstamplifier having an input connected to said first transducer element andan output connected to said second transducer element, a thirdelectromechanical transducer element fixedly mounted on one of thevibratory reeds of said other Ushaped mechanical vibrator, a fourthelectromechanical transducer element fixedly mounted on the othervibratory reed of said other U-shaped mechanical vibrator, a secondamplifier having an input connected to said third transducer element andan output connected to said fourth transducer element, a mechanicalcoupling connecting said base portions of said vibrators to effectamplitude modulation of the frequency f2 at the frequency f1, and a lowpass filter connected to said output of said first amplifier to providethe low frequency f0 f2-f1.
 2. A low frequency oscillator according toclaim 1, wherein each of said transducer elements is a piezoelectricelement.
 3. A low frequency oscillator according to claim 1, comprisinga third amplifier connected to said low pass filter for receiving andamplifying oscillations at the low frequency f0.